Oh Twinkie, I'm so sorry. You've moved me to tears too, also because my 11wk old is called Miles and I can't imagine having to go through with him what your sister did with her little one.
Of course you need to grieve etc - not only did you support your sister wonderfully through it, but Miles was your nephew, a very close relative, that I'm sure you had your own feelings for him inside. I don't really know what to say, but I think it is very much OK to speak about Miles, he was their son and they saw him and he was lovely. Your sister and husband won't want him to be forgotten or not spoken about - especially by you, who were right there with them at the end.
I think you are amazing and very strong and I'm so sorry that your family has had such a sad experience. Thinking of you.